what exactly does g/m2 mean?

i understand that its grams per meter squared, but what exactly does that translate to yield wise?

for example if i had a 1.5 foot tall plant with a 650 g/m2 potential yield, what am i looking at here?

I believe 650 g/m2 usually means that you can potentially get 650 grams for every 1m X 1m area in a SOG grow under ideal conditions (lighting, nutes, etc.). In a 1 sq meter area SOG you might be growing up to like 36 plants, which is about 18 g/plant. That is not to say you can't get the same yield from 4 plants in 1 sq meter, it would just require a longer veg time.

Best to just use these numbers for comparison, not as a guide to what you'll yield in your particular setup.